STROMATOLITE-THROMBOLITE ASSOCIATIONS IN A MODERN ENVIRONMENT, LEE STOCKING ISLAND, BAHAMAS

Columnar buildups found in a tidal channel off Lee Stocking Island, Ex uma Gays, Bahamas, have been interpreted as modern giant stromatolites growing in a subtidal normal marine environment. However these organi cally-formed columns reveal three discernible internal structures: (1) prokaryotic stromatolites comprised of alternating layers of coarse-g rained ooids and peloids, and fine micrite that formed exclusively by microbial activity; (2) eukaryotic stromatolites comprised of microbia lly-induced micritic layers alternating with detrital layers accumulat ed bound, and cemented by eukaryotic algae; and (3) thrombolites displ aying irregular, clotted fabrics and formed by microbes, algae, and me tazoans. Phanerozoic thrombolites, in contrast, have been interpreted as unlaminated stromatolites constructed by cyanobacteria. Eukaryotic organisms overgrow all of the columns at present. Thus, the contempora neous formation of prokaryotic stromatolites, eukaryotic stromatolites , and thrombolites under identical conditions within the present envir onment appears unlikely. We suggest that the prokaryotic stromatolites represent forms that began to develop in. an intertidal setting with the Holocene flooding of the Great Bahama Bank. The thrombolites, howe ver began to form under the present, normal-marine subtidal conditions . The eukaryotic stromatolites represent intermediate forms between pr okaryotic stromatolites and thrombolites. There is evidence for a grad ual change from stromatolite to thrombolite reefs associated with risi ng sea-level. With the deepening, there would have been a decrease in salinity, an increase in energy, and possibly an increase in nutrient supply; all factors that favor thrombolite growth. We propose that the co-existing stromatolites and thrombolites found off Lee Stocking Isl and did not grow contemporaneously, but reflect a response to changing environmental controls with changing sea-level.